In optical communication, in order to compensate for transmission loss of optical fibers, or insertion loss of optical components such as AWG (Arrayed Waveguide Grating), optical amplifiers having low noise and high gain are very essential. Current excitation type semiconductor optical amplifiers (SOAs) differ from Er-Doped Fiber Amplifiers (EDFA) in that there is no need for a pumping laser, so the optical amplifier is compact and inexpensive. Particularly, in recent years, semiconductor optical amplifiers have received a lot of attention because they are compact and can be integrated into small optical circuits such as AWG. In the initial stages of development of semiconductor optical amplifiers, the semiconductor optical amplifiers had lower saturation output power and inferior noise figure (NF) characteristics when compared with EDFA, however, as development has progressed in recent years, semiconductor optical amplifiers have been reported that are comparable to EDFA from the aspect of saturation output and noise figure (for example, refer to Patent document 1, and Non Patent document 1).
First, the saturation power, gain factor and net gain, which are parameters that determine the light output of a semiconductor optical amplifier will be explained. The light output of a semiconductor optical amplifier depends on the saturation power parameter. A semiconductor optical amplifier having a large saturation power is capable of achieving large light output because it is difficult for gain saturation to occur even when the input power is amplified and the output power becomes large. Taking the saturation power to be Isat, Isat can be expressed by Equation (1) below.Isat=hν/(ΓAgτ)×W×d  (1)
In equation (1), h is Planck's constant, Γ is the optical confinement factor in waveguide structure that performs optical amplification, Ag is the differential gain factor, τ is spontaneous carrier lifetime of light, W is the width of the active core layer, and d is the thickness of the active core layer.
By taking the gain factor of the active core to be g(n) (n is the carrier density), g(n) can be expressed by Equation (2) below.g(n)=Ag×(n−n0)  (2)Wherein, In Equation (2), n0 is the transparent carrier density.
The net gain G, which is the gain per unit length that includes the waveguide dependency or optical loss of the semiconductor optical amplifier can be expressed by Equation 3 below.G=Γ×g(n)−α=Γ×Ag×(n−n0)−α  (3)
Wherein, in Equation (3), α is the loss coefficient of light.
Next, the design of a semiconductor optical amplifier for obtaining high efficiency and high output power characteristics will be explained. FIG. 12 is a diagram illustrating the profile in the lengthwise direction of a semiconductor optical amplifier having a preferred net gain G=Γ×g(n)−α in order to obtain high efficiency and high output power characteristics. Note, on the horizontal axis, “Input” is the side where the signal light having small strength is input, and “Output” is the side where the signal light that has been amplified and whose strength has become large output. As illustrated in FIG. 12, in a preferred profile, on the input side where the strength of the signal light is low, the width of the active core layer is narrow, the carrier density is high, the optical confinement factor is high and the net gain is high. On the other hand, on the output side where the strength of the signal light is large, by making the width of the active core layer large, the optical confinement factor low and the saturation output Isat large in order for the light output not to saturate, the net gain becomes low.
Conventionally, in order to achieve the net gain profile as illustrated in FIG. 12, a semiconductor optical amplifier having a tapered waveguide structure has been widely used. FIG. 13 is a diagram that illustrates the waveguide structure in the width direction of a conventional semiconductor optical amplifier. As illustrated in FIG. 13, this semiconductor optical amplifier 200 outputs input signal light IL, which was input from an input section 200a, from an output section 200b as an output signal light OL. This semiconductor optical amplifier 200 is such that both sides in the width direction of a mesa shaped active core layer 30 have an embedded mesa waveguide structure that is embedded by a current blocking semiconductor layer 31 that functions as a clad section. Furthermore, the active core layer 30 comprises a narrow width section 30a on the side of the input section 200a having a relatively narrow width, and a wide width section 30b comprising a tapered section 30ba having a tapered shape such that it increases in width going toward the output section 200b side, and a wide section 30bb having a wide width. In FIG. 13, reference number D2 indicates the optical electric field strength distribution of the input signal light IL in the width direction in the narrow width section 30a. With the construction described above, this semiconductor optical amplifier 200 is such that on the input section 200a side the carrier density, optical confinement factor and net gain are high, and on the output section 200b side, the wide width section 30b increases the saturation output Isat and lowers the net gain.